This invention relates generally to storage tanks for hot fluids and, more particularly, to hot water storage tanks such as water heaters.
In the prior art, a storage-tank water heater replaces hot water withdrawn from the top of the tank with cold water delivered to the bottom of the tank. Because typical tank heating elements cannot heat the water as fast as it is withdrawn, cold water will eventually fill the tank. Even before the tank is filled with cold water, the incoming cold water mixes freely with the heated standing water in the tank thereby causing deterioration of the tank's water temperature. This mixing is partially the result of the currents generated by the inward flow of cold water, by the outward flow of hot water, and by the convection currents established within the tank.
Because of this mixing, hot water delivered by a typical hot water heater will gradually decrease in temperature while water is being withdrawn, only a small amount of high temperature water is delivered relative to the tank's total capacity. The hot water volume delivered to the outlet above a specified temperature can obviously be extended by increasing the size of the tank or by increasing the BTU input of the heating elements or gas/oil burner. The temperature of hot water at the outlet can also be maintained by preventing the mixing of hot and cold water within the tank.
Attempts have been made in the past to contain and control the mixing of hot and cold water by providing separate chambers within the tank for cold and hot water. Miller U.S. Pat. Nos. 2,833,273 and 3,244,166 employ separate mixing chambers within the tank at the inlet. Gulick U.S. Pat. No. 2,207,057 uses a small baffle over the inlet to control mixing, while Downs et al. U.S. Pat. No. 3,987,761 employs a baffle plate with large openings. Hammersley U.S. Pat. No. 3,062,233 simply uses a small inverted inlet cover. Fox U.S. Pat. No. 787,909 and Andrews U.S. Pat. No. 4,390,008 show the use of a vertically movable barrier. In Schauer, Jr. U.S. Pat. No. 2,809,267, a braided tube is attached to the cold water inlet located adjacent the tank bottom to control the turbulent introduction of cold water into the tank and in an attempt to maintain the stratification of hot water above cold water.
In substantially different constructions employing the concept of compartmentalization, Jacoby U.S. Pat. No. 2,625,138 divides the tank into a plurality of separate vertical layers by using numerous horizontal baffles and Pruitt U.S. Pat. No. 2,311,469 shows a fuel burner in which several secondary combustion chambers stratify the water in the storage tank.
McAlister U.S. Pat. No. 4,436,058 attempts to minimize convection tendencies by confining water in numerous capillary type conduits stretched between the tank bottom and the tank top. Schuell U.S. Pat. No. 1,689,935 attempts to obtain constant temperature of water by continuously varying the energy input to the tank by using a feedback control system involving a thermostat.
While these prior art designs tried to reduce flow created by the usual high velocity of incoming cold water and tried to separate hot and cold water layers, none have taken note of the existence of possible convection currents and, thus, none limit the formation of these thermal currents in the tank and concurrently preserve the smooth horizontal boundary layer between hot and cold water within the tank. Further, these convection thermal currents are believed to flow primarily along the smooth side surfaces of the tank. In pressurized tanks, these currents are enhanced by the smooth inner surface of the curved top, the "domed" top being common in pressure tanks because of their structural strength. These closed loop currents greatly enhance the mixing of hot and cold water. My U.S. Pat. Nos. 4,632,065 and 4,739,728 attempt to stop mixing caused by these convection currents.
Convection currents are believed to be generated by physical turbulence resulting from high velocity of inrushing cold water and by thermal imbalance created by the localized dumping of cold water into the tank. None of the above patents is concerned with reducing or minimizing convection currents near the cold water inlet. Further, none of the above patents is concerned with reducing or minimizing convection currents resulting from thermal imbalance created due to localized dumping of cold water. If cold water is introduced uniformly throughout a horizontal cross section of the heater tank, these convection currents can be minimized.
In the above-referenced patents, mixing of cold water is prevented in a more active manner by presenting a physical obstruction to convection currents. The new constructions disclosed herein prevent mixing in a more passive manner by foiling convection currents by use of a diffuser inlet dip tube. In contrast to the single point inlet presently employed in heaters, the diffuser inlet introduces water evenly across a horizontal cross section of the tank. In this way, the possibility of establishing closed loop convection currents is minimized. In effect, this foils the convection currents that would otherwise be established within the tank which cause mixing. This aspect of minimizing mixing by use of an inlet diffuser which introduces cold water uniformly across a horizontal cross section is novel, unique and very cost effective.